This application relates to polymer gels, in particular methods of processing hydrocarbon gels.
In today's modern electrical and electronic devices, as well as in other uses such as fiber optic connections, sealants are often used for insulation, for protection against water, corrosion and environmental degradation, optical index matching, and thermal management. Prior to now, a number of sealants including gels have been known, however, processing gels in a cost effective, efficient, and effective manner has been a challenge.
As technology progresses, sealants will be subjected to increasingly higher temperature environments and more demanding performance requirements. There has been, and there presently exists, a need for high performance sealants to meet these demands. For example, there is an increasing need for high service gel sealants for use in outdoor energy transmission applications and for use near engine compartments. As the need for high performance sealants increase, so also, does the need for improved processing methods. In particular, improved processing methods for crosslinked gels are needed.
Gels, for example, have been used as sealants with relative success in certain applications due to their unique properties. Gels may have a lower hardness than rubber and can seal and conform under adequate compression. Gels may also be more elastic than mastics. Other advantages of gels are known in the art. For example, gels, when used as sealants, may be removed and re-entered more easily due to elastic recovery of the gel. For further example, relatively little force is required to change the shape of a soft gel sealant.
Solid particulates have been added to alter a gel's properties. However, one of the problems with flame retarding a soft gel is that the addition of solid particulate fillers leads to hardening and produces a gel with poor sealing properties. Other disadvantages of gels are known in the art.
One class of gels used as a sealant is thermoplastic elastomer gels (TPEGs). Certain TPEGs have advantages over other classes of gels such as silicone gels, polyurethane gels, and polybutadiene gels. For example, silicone gels may have a higher cost compared to TPEGs, a silicone gel's dielectric breakdown voltage may be adversely affected by humidity, and low surface energy silicone oils can leak or evaporate out of the gel and spread over electrical contact points leading to problematic insulation barriers. Problems with polyurethane and polybutadiene gels include, for example, hydrolytic instability of the crosslinked network; and degradation and hardening with aging. In addition, environmental concerns regarding certain non-TPEGs have led to an increased interest in developing gels with enhanced safety profiles while achieving sufficient or enhanced properties.
TPEGs have provided many years of reliable in-field performance for applications requiring a low maximum service temperature of approximately 70° C. TPEGs have been made that comprise a styrene ethylene/butylene styrene (“SEBS”) triblock copolymer swollen with a mineral oil softener. While the thermoplastic nature of these gels allows for easy production, it limits the upper service temperature due to creep and flow as in-field ambient temperatures approach the styrene glass transition. Research has been aimed at increasing the upper service temperature of these gels through chemically crosslinking the gel network in order to form a thermoset gel structure. For example, oil-swelled acid/anhydride modified maleic anhydride SEBS gels have been covalently crosslinked using small molecule crosslinkers like di- and triamines, European Patent Publication No. EP 0879832A1, as well as with some metal salts, D. J. St. Clair, “Temp Service,” Adhesives Age, pp. 31-40, September 2001. Crosslinked polymers are known to increase thermal stability, toughness, and chemical resistance compared to their base, or uncrosslinked polymers. However, crosslinked polymers are also known to often be intractable, making them difficult to reprocess or recycle.
Thermoset gels, in contrast to thermoplastic gels, are not plasticized upon heating due to the chemically crosslinked network within the gel. Thermoset gels include silicone gels and other types of gels that are used in many industries. Thermoset gels may provide the advantage of high service temperatures imparted by a chemically crosslinked network. However, processing of silicone gels is very different from processing TPEGs. Processing silicone gels typically requires the use of sensitive transition metal catalysts to generate the crosslinked gel network. Typically a two-part system is employed, where a first part includes silicone oil, a vinylsilane polymer, and a platinum catalyst. A second part includes the silicone oil and a silylhydride crosslinker. The first and second parts are then typically dispensed into the part and the material gels upon reaction between the vinylsilane and silylhydride.
Traditional thermoplastic elastomer gels are plasticized by heat and can be easily processed when molten. Styrenic block copolymers are typically used in TPEGs and these polymers form a physically crosslinked network of glassy styrene domains within the mineral oil extender fluid. At temperatures below the Tg of styrene, the gel is stable and does not flow, but raising the temperature above the styrene Tg will cause the gel to flow. These thermoplastic properties allow for easy processing of these gels into a usable part.
Typically, the components of the gel are mixed in a large drum using high shear and temperatures in the range of 177-220° C. Alternatively, the gel components are compounded using an extruder or Banbury mixer and then dispensed while molten into a large drum. Once the gel cools and sets it is sent to a manufacturing facility where a drum melter with a heated piston pushes into the drum, melting the contacted gel layer and subsequently dispensing the molten gel into the housing of a part.
A number of problems with processing gels are known in the art. For example, when processing a crosslinked gel that contains both a crosslinked polymer network with a miscible fluid, the miscible fluid may diffuse out of the gel.
U.S. Pat. No. 6,207,752 to Abraham et al. relates to low oil swell carboxylated nitrile rubber-thermoplastic polyurethane vulcanizate compositions. The nitrile rubbers of Abraham contain pendant carboxyl groups that can be crosslinked. The patentees report unexpectedly discovering that a processing aid can improve the processability of the compositions. The patent lists a number of processing aids including maleated polyethylene, maleated styrene-ethylene-butene-styrene-block copolymers and maleated styrene-butadiene-styrene-block copolymers, and maleated ethylene-propylene rubber.
U.S. Pat. No. 6,756,440 to Hase et al. relates to a fire resistant resin composition, a method of making the resin composition and an electrical wire comprising the composition. The composition has a halogen-free propylene resin containing propylene as a monomer component, a halogen-free styrene-based thermoplastic elastomeric resin modified with an unsaturated carboxylic acid or a derivative of such an acid, and a fire resistant metal hydroxide.
U.S. Published Patent Application No. 2002/0065356 to Crevecoeur et al. relates to flame retardant polymers with a condensation polymer, a halogen-containing styrene polymer, a polymer derived from aromatic vinyl monomer, and elastomeric polymer segments. The polymers derived from aromatic vinyl monomers may be crosslinked.